Grid controlled x-ray generator with magnetic field



Jan. 9, 1968 .1. T. PERRY 3,363,131

GRID CONTROLLED X-RAY GENERATOR WITH MAGNETIC FIELD Filed March 4, 1966 2 Sheets-Sheet 1 JOHN TPERRY //VVE/VTOR BY BUG/(HORN, BLORE, KLAROU/ST a SPAR/(MAN ATTORNEYS Jan. 9, 1968 J. T. PERRY 3,363,131

GRID CONTROLLED X-RAY GENERATOR WITH MAGNETIC FIELD Filed March 4, 1966 2 Sheets Sheet 2 Hog 5) BUC/(f/OEW, BAG/PE, KLAROU/ST 8 SPAR/(MAN ATTORNEYS Patented Jan. 9, 1968 3,363,131 GRID CONTROLLED X-RAY GENERATQR WITH MAGNETIC FEEL!) John T. Perry, Melrose Park, BL, assignor to Dunlee Corporation, Beliwood, lll., a corporation of Illinois Filed Mar. 4, 19fi6, Ser. No. 531,807 Claims. (Cl. 313162) The present invention relates to an X-ray generator and more particularly to an X-ray generator having a grid therein for controlling electron emission.

A particular object of the present invention is to improve the operating characteristics of a thermionic emission cathode type X-ray generator. To increase the efficiency of such a generator, and to effect close control Over the timing of exposures, it has been proposed heretofore to provide such generators with a grid interposed between the cathode and the anode for controlling the electron flow from the cathode filament to the anode. Such a generator is shown in Atlee Patent, 2,686,884. The construction shown in such patent is useful at low operating potentials or even at high operating potentials if the grid is operated only at negative potential with respect to the cathode. However, in operating high potential generators, that is, those of the order of 100 pkv. or higher for medical radiography and the like, it is desired to drive the grid positive on occasions in order to obtain maximum electron flow between the cathode and the anode. However, as soon as the grid is made positive it tends to attract a portion of the electron flow from the filament which gives rise to three problems. First, any electrons striking the grid will tend to heat the same and the grid may be overheated to destruction. Second, because of the electron adsorption by the grid a part of the available electrons emitted from the cathode filament can not be utilized for the generation of X-rays from the anode. Third, since the grid will draw current it requires driving power to establish the positive potential.

Accordingly, it is an object of the present invention to provide a grid controlled X'ray generator so constructed as to minimize the attraction of the electron fiow to such grid from the electrons emitted by the filament.

Further objects and advantages of the invention will become more apparent hereinafter.

In accordance with an illustrated embodiment, an X- ray generator of the present invention comprises a cathode including an elongate filament for the emission of elec trons and an anode for the generation of X-rays. A control grid is positioned between the filament and the anode which grid comprises a plurality of laterally spaced apart wires extending perpendicularly to the longitudinal direction of the filament. Magnet means are provided in the cathode for providing a magnetic field in the area of the grid with the fiux lines parallel to the grid wires, the effect of such field being to provide paths for electrons emitted from the filament such that they will pass between the grid wires rather than to strike the grid and be captured thereby.

For a more detailed description of the invention reference is made to the accompanying drawings and to the following description of the invention.

In the drawings, FIG. 1 is a longitudinal section view taken through an X-ray generator constructed in accord ance with the present invention;

FIG. 2 is an enlarged view of the cathode taken along line 2 2 of FIG. 1;

FIG. 3 is a sectional view of the cathode taken along line 33 of FIG. 2;

FIG. 4 is a longitudinal sectional view of the cathode taken along line 44 of FIG. 2;

FIG. 5 is an enlarged fragmentary view of a portion of a cathode as shown in FIG. 3 showing details of the construction thereof;

FIG. 6 is a schematic view showing the paths of electrons escaping from the filament in a conventional grid type "enerator;

FIG. 7 is a view showing paths of electrons escaping from a grid provided with the improvement of the present invention; and

KG. 8 is a fragmentary sectional view of a modified cathode construction.

The embodiment of the invention illustrated in the drawings comprises an X-ray generator or tube 10 having an anode 12 and a cooperating cathode 14 mounted in spaced apart relation within an elongated envelope 16 formed of glass or other suitable material. The anode 12 may be of any suitable construction and suitably mounted in one end of the envelope 16 and is provided with a stem 13 extending outwardly from the envelope for the purpose of providing electrical connection to the anode.

Referring more particularly to FIGS. 2 to 5, the cathode 14 of the present invention comprises a body 22 formed of a non-magnetic material such as Monel having a focusing cup 24 formed in the face thereof. The body 22 is mounted within a sleeve 26, also preferably of Monel, and which sleeve surrounds and is secured to a mounting disc 28 sealed to a tubular mounting member 36 which in turn is sealed by a glass-to-metal seal 31 to the reentrant end 32 of the envelope 16. A cup-shaped supporting member 34- is also secured to the disc 28, over which member the sleeve 26 is fitted. The sleeve 26 defines a skirt 35 which extends over the mounting member 30 and is provided with a groove for retaining a getter ring 36 as more particularly described in US. Patent 2,502070, Atlee. The body 28 is provided with a threaded opening in its outer surface for the receiving of a cooperatively threaded stem 38 by means of which the body 28 and hence the cathode 14 may be grounded and by means of which the generator may also be supported.

Mounted within the focusing cup 24 is an elongate filament of Wolfram or other suitable material supported at one end by a conductor 42 which extends outwardly of the envelope through the supporting disc 28 through a seal 44. The opposite end of the filament 40 is connected by a ocnductor 46 to a projection 48 of rectangular cross section on the rear side of the cathode body 22. Extending transversely of the sleeve 26 with respect to the filament 4i) is a permanent magnet 59, which may be of ferrite or other suitable material magnetized so that its poles are at its opposite ends and thus are spaced on opposite sides of the axis 52 of the cathode and opposite sides of the filament 40. A pair of pole pieces 54, 56 are arranged one with each of the poles of the magnet 59. The pole piece 54 comprises an arm 58 engaging one end of the magnet and extending forwardly toward the face of the cathode, and an end piece 69 extending inwardly toward the filament and terminating in a plane face 62 parallel to the axis of the filament but spaced slightly forwardly of the filament 40 and to one side thereof. Similarly the pole piece 56 comprises an arm 64 extending forwardly from the magnet 50 to an end piece 56 extending inwardly and terminating in a plane face 58 parallel to the axis of the filament 40 and spaced opposite the face 62 to the other side of the filaient. As will be apparent such arrangement will form a magneitc field between the faces 62, 68 just forwardly of the filament 4t} with the lines of force of such field extending normal to the longitudinal axis of the filament. The function of this field will be described subsequently.

A grid 80 is mounted on said cathode comprising a plurality of parallel laterally spaced apart co-planar wires 32 extending normal to the axis of the filament 46). The wires 82 are positioned just slightly rearwardly of or in the plane of the forward edges of the pole faces 62, 63 and are clamped in position, as best shown in FIG. 5, between a rectangular cup like retainer member 84 and a metal clamping plate 86 both of which are formed of a nonmagnetic metal. The member 84 and plate 86 are secured to the body 22 by bolts 88 extending through the body but insulated therefrom by insulated spacers 9t 92. Nuts 96 are threadedly received on' the bolts 38. The member 84 is insulated from the body 22 by a layer of insulating material 98 whereby the grid 80 is electrically insulated from the filament 40. The member 84, plate 86 and insulating material 98 are provided with registering apertures such as the aperture 1% of the plate 86 (see FIG. '2) so that electrons emitted by the filament 40 may pass through such apertures to the anode 14.

To permit an electrical potential or bias to be applied to the grid 80 one of the bolts 83 is connected to a conductor 106 extending outwardly of the envelope through a conventional seal 108 arranged with the supporting disc 28.

The function of the arrangement described in maximizing the number of electrons leaving the filament and reaching the anode can best be understood by reference to FIGS. 6 and 7. Referring first to FIG. 6, in such view is shown the effect of the potential field about a grid wire 82 in a conventional grid type generator in which it will be assumed the anode is at a positive potential of 100 kilovolts and the grid at a positive potential of 600 volts. The filament is indicated in FIG. 6 by a single line 40 and the potential gradient from the cathode to- Ward the anode by the lines 198, the potentials being indicated in volts. The paths of electrons leaving the filament in the area between a pair of grid wires 82 are indicated by the lines A, A. Such electrons will be accelerated toward the anode, the increase in voltage along the paths of the electrons being approximately linear. Electrons leaving the filament in the area between grid wires Will attain sucfliient velocity to overcome any attraction from the grid and will escape the field of the grid soas to reach the anode.

However, electrons leaving the filament in the area behind a grid wire in a critical area indicated by the brackets 110 are otherwise influenced, the paths of such electrons being indicated by the lines B, B. An electron leaving the critical area behind a grid wire 82'is initially accelerated toward the anode. However, as it approaches a grid wire 82 it encounters the field surrounding the 'grid and will be attracted to the grid wire 82. Such electrons create an undesirable grid current and, of course, since they fail to reach the anode reduce the overall efficiency of the generator as well as create heating problems with the grid.

Referring now to FIG. 7, the situation is the same as in FIG. 6 except that we shall here consider the effect of the imposition of the magnetic field between the pole faces 62, 68, such field being indicated by the dotted area 116. The flux lines of such field will be perpendicular to the plane of the drawing. In this situation electrons leaving the filament 40 will be subject not only to the force of the accelerating potential and the field about each grid wire, but also to the force of the magnetic field 116. The combined action of these fields causes electrons emitted from the cathode surface to follow cycloidal paths, the radius of which is proportional to the intensity of the electrostatic field and inversely proportional to the square of the magnetic field intensity.

The result of this effect on the path of an electron leaving the filament in the area between grid wires is shown at C. Such electron is subjected not only to the magnetic field forces but also is subjected to acceleration in the electrostatic field between the anode and cathode. As a result it travels in a path of ever increasing radius until it finally escapes the magnetic field 116. An electron leaving the filament behind a grid wire 82 will, on the other hand, not be subjected to as rapid acceleration as a consequence of which it will follow a cycloidal path to strike the filament 46" at a point clear of a grid wire 82, as shown by the path C. It will, of course, then be repelled by the cathode and again accelerated toward the anode. The electron will again tend to follow a cycloidal path, but this time enter a field of higher accelerating potential and which is of sufiicient strength to increase the radius of curvature to such value that the electron escapes the magnetic field and thence is accelerated directly to the anode. I

As will be apparent, the strength of the field 116 should be selected depending upon the potential between the cathode and anode and the grid potential.

A modified grid arrangement wherein the grid is maintained at cathode potential is shown in FIG. 8. In such arrangement the grid consists of parallel wires 82 clamped between a metal retainer member 84 and metal clamping plate 86', the retainer member being in this instance in direct contact with the body 22. Screws secure the members 84 and 86 in position. The magnetic field functions in a similar manner in the embodiment of FIG. 8 to facilitate the passage of electrons through the grid.

Having illustrated and described a preferred embodiment of the invention it should be apparent to those skilled in the art that the invention permits of modification in arrangement and detail. I claim all such modifications as come within the spirit and scope of the appended claims.

I claim:

1. An electron discharge device comprising? an anode and a cooperating cathode,

said cathode comprising an elongated electron emission element,

a control grid disposed in the electron fiow path between said cathode and anode,

said grid comprising a plurality of parallel conductors extending at right angles to the longitudinal axis of said electron emission element,

and means for forming a magnetic field between said electron emission element and said grid with the lines of force of such field parallel to said conductors.

2. An electron discharge device as set forth in claim 1 wherein said grid is electrically insulated from said electron emission element, and

means are provided for electrically biasing said grid with respect to 'said electron emission element.

3. An X-ray generator comprising:

an anode,

a cathode spaced from said anode and comprising a body of nonmagnetic metal,

said body having an elongated electron focusing cup formed therein,

an elongated filament mounted in said focusing cup,

a permanent magnet positioned behind said body with respect to said anode,

a pair of pole members of soft iron engaged one with each of the poles of said magnet and extending around said body and terminating along the opposite sides of said focusing cup whereby a magnetic field is formed in front of said filament with the lines of force extending normal to the longitudinal axis of said filament, and a grid means positioned forwardly of said focusing cup and comprising a plurality of wires extending normal to said filament axis and spaced laterally with respect to one another and positioned to be in said magnetic field. d. An X-ray generator as set forth in claim 3 including means electrically connecting said grid means to said 75 body.

5. An X-ray generator as set forth in claim 3 including No references cited. means mounting said grid means in electrically insulated relation with respect to said filament, said body and said D AVID GALVIN, Primary Examiner pole members,

and means for electrically biasing said grid means with 5 D. J. GALVIN, Assistant Examiner.

respect to said filament. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING: AN ANODE AND A COOPERATING CATHODE, SAID CATHODE COMPRISING AN ELONGATED ELECTRON EMISSION ELEMENT, A CONTROL GRID DISPOSED IN THE ELECTRON FLOW PATH BETWEEN SAID CATHODE AND ANODE, SAID GRID COMPRISING A PLURALITY OF PARALLEL CONDUCTORS EXTENDING AT RIGHT ANGLES TO THE LONGITUDINAL AXIS OF SAID ELECTRON EMISSION ELEMENT, AND MEANS FOR FORMING A MAGNETIC FIELD BETWEEN SAID ELECTRON EMISSION ELEMENT AND SAID GRID WITH THE LINES OF FORCE OF SUCH FIELD PARALLEL TO SAID CONDUCTORS. 